1. Field of the Invention
The present invention relates to an optical amplifying apparatus that has optical amplifiers corresponding to respective wavelength bands and can almost equalize optical powers of the respective wavelength bands at a point that an optical signal reaches after traveling a predetermined distance by controlling the outputs of the respective optical amplifiers, as well as to a related optical sending apparatus. The invention also relates to an optical transmission system using these apparatuses as an optical repeater. The invention further relates to an optical amplifying method in an optical amplifying apparatus that has optical amplifiers corresponding to respective wavelength bands.
To construct future multimedia networks, ultra-long-distance, large-capacity optical transmission systems are now required. The wavelength-division multiplexing (hereinafter abbreviated as WDM) is now researched and developed as a scheme for realizing such increase in capacity, because of its capability of effectively utilizing wide bandwidth and large capacity of optical fibers and other advantages.
In particular, with demand for enhancement of the WDM in bandwidth and the number of usable wavelengths, optical amplifying apparatuses for amplifying WDM optical signals that are increased in bandwidth and the number of wavelengths are now researched and developed extensively.
2. Description of the Related Art
An optical amplifying apparatus for amplifying WDM optical signals that are increased in bandwidth and the number of wavelengths was reported (“Trinal-wavelength-band WDM transmission over dispersion-shifted fiber”, Jun-ichi Kani et al., 1999 IEICE General Conference).
Referring to FIG. 23 that is drawn based on the above report, 16 laser diodes (hereinafter abbreviated as LDS) 150-1 to 150-16 emit laser beams having wavelengths that correspond to channel-1 to channel 16 of the S+ band, respectively. The emitted laser beams are input to an arrayed waveguide grating (hereinafter abbreviated as AWG) 151-1. The AWG 151-1 generates WDM light by wavelength-multiplexing the laser beams of channel-1 to channel-16. The WDM light is input to a Mach-Zehnder interferometer type optical modulator (hereinafter abbreviated as “MZ modulator”) 152-1, where it is modulated with information to be transmitted and thereby converted into a WDM optical signal. The WDM optical signal is input to a thulium-doped fiber amplifier (hereinafter abbreviated as TDFA) 153. Being a rare-earth-element-doped fiber amplifier that amplifies light in a 1,450-nm band, the TDFA 153 can amplify an S+-band WDM optical signal. The amplified WDM optical signal is input to a wavelength-multiplexing coupler (hereinafter referred to as “WDM coupler”) 156 that is a dielectric multilayer optical filter.
A C-band WDM optical signal is generated by a block that is similar to the above block and is composed of LDs 150-17 to 150-32, an AWG 151-2, an MZ modulator 152-2, and an erbium-doped fiber amplifier (hereinafter abbreviated as EDFA) 154 that is provided instead of the TDFA 153 and performs amplification in a 1,550 nm band. The C-band WDM optical signal is input to the WDM coupler 156.
An L-band WDM optical signal is generated by a block that is similar to the above block and is composed of LDs 150-33 to 150-48, an AWG 151-3, an MZ modulator 152-3, and a gain-shifted erbium-doped fiber amplifier (hereinafter abbreviated as GS-EDFA) 155 that is provided instead of the TDFA 153 and performs amplification in a 1,580 nm band. The L-band WDM optical signal is input to the WDM coupler 156.
The WDM coupler 156 generates a three-wavelength-band WDM optical signal by wavelength-multiplexing the S+-band, C-band, and L-band WDM optical signals. The three-wavelength-band WDM optical signal is sent to an optical transmission line, that is, a dispersion-shifted fiber (hereinafter abbreviated as DSF) 157.
In the above optical transmission system, WDM optical signals that are channel-allocated to the wavelength bands of the S+ band (1,450-1,490 nm), the C band (1,530-1,570 nm), and the L band (1,570-1,610 nm), respectively, are generated, amplified by the rare-earth-element-doped fiber amplifiers on a wavelength band basis, and wavelength-multiplexed into a three-wavelength-band WDM optical signal, which is sent to the optical transmission line.
Incidentally, it is known that crosstalk occurs between WDM optical signals traveling through an optical transmission line owing to nonlinear optical phenomena such as stimulated Raman scattering, four-wave mixing, and cross-phase modulation.
In particular, the stimulated Raman scattering makes optical powers of respective channels non-uniform because it causes shorter-wavelength optical power to be transferred to a longer-wavelength side through interaction with optical phonons in the optical transmission line. This causes a gain gradient and hence deteriorates the optical signal-to-noise ratios (hereinafter abbreviated as “optical SNRs”) of WDM optical signals of shorter-wavelength channels.
Where a WDM optical signal is in a frequency band of 15 THz having Raman gain, the proportion D of optical power that is removed from the shortest-wavelength channel of the WDM optical signal is given by                    D        =                              ∑                          i              =              1                                      N              -              1                                ⁢                                           ⁢                                    (                                                λ                  ⁢                                                                           ⁢                  i                                                  λ                  ⁢                                                                           ⁢                  O                                            )                        ×                          (                                                Pi                  ⁢                                                                           ⁢                  γ                  ⁢                                                                           ⁢                  iLe                                                  2                  ⁢                  Aeff                                            )                                                          (        1        )            where N is the number of channels and λi, Pi, and γi are the wavelength, optical power, and Raman gain coefficient, respectively, of an i-th channel. Le is the effective length of the optical transmission line and is given by Le={1-exp(−α)}/α where α is the loss coefficient of the optical transmission line. Aeff is the effective core cross section of the optical transmission line.
In general, the Raman gain coefficient is triangle-approximated and given by                               γ          ⁢                                           ⁢          i                =                                            i              ⁢                                                           ⁢              Δ              ⁢                                                           ⁢              f              ⁢                                                           ⁢              γ              ⁢                                                           ⁢              p                        ⁢                                                                       1.5            ×                          1              ′                        ⁢                          0              13                                                          (        2        )            where Δf is the space between channels and γp is the peak gain coefficient that is the maximum value of Raman gain coefficients that are obtained by the triangle approximation.
Formulae relating to the stimulated Raman scattering including the above equations are described on pp. 276-278 of “Optical Fiber Communication Technology” (supervised by Yoshihiro Konishi, The Nikkan Kogyo Shinbun, Ltd.).
It is known that if a 32-wave WDM optical signal is transmitted over a certain distance through an optical fiber, stimulated Raman scattering causes part of the optical power of channel-1 to be transferred to longer-wavelength channels and hence causes a gain gradient in the WDM signal. That is, it is known that a gain gradient due to stimulated Raman scattering occurs in a WDM optical signal in a single wavelength band.
Incidentally, it is calculated that the range of the interaction of the stimulated Raman scattering in a wavelength band around 1,550 nm covers a wide wavelength band of 130 nm or more. Therefore, when a three-wavelength-band WDM optical signal whose channels are set in three wavelength bands are transmitted over 100 km in the optical transmission system of FIG. 23, it is expected that at point X, which is the point where the transmission ends, the optical SNRs deteriorate because stimulated Raman scattering causes part of the optical power of the S+ band that is a shorter-wavelength band to be transferred to the C band and the L band that are longer-wavelength bands.
Based on the above understanding, a measurement was performed to evaluate how the stimulated Raman scattering influences a two-wavelength-band WDM optical signal that is transmitted in both of the C band and L band.
Referring to FIG. 17, 32 LDs 120-1 to 120-32 emit laser beams having wavelengths that correspond to channel-1 to channel-32 of the C-band, respectively. The emitted laser beams are input to an AWG 121-1, where they are wavelength-multiplexed into WDM light. The WDM light is input to an EDFA 122 and amplified there. The amplified WDM light is input to an attenuator (hereinafter abbreviated as ATT) 123 that attenuates optical power. The WDM light whose optical power has been attenuated to a predetermined level is input to a WDM coupler 126.
L-band WDM light is generated by a block that is similar to the above block and is composed of LDs 120-33 to 120-64, an AWG 121-2, a GS-EDFA 124 that is provided instead of the EDFA 122, and an ATT 125. The generated L-band WDM light is input to a WDM coupler 126.
The WDM coupler 126 generates a two-wavelength-band WDM light by wavelength-multiplexing the C-band and L-band WDM light beams, and sends it to a single-mode fiber (hereinafter abbreviated as SMF) 127.
After being transmitted through the SMF 127 over 80 km, the two-wavelength-band WDM light is input to an optical spectrum analyzer 128 that measure the wavelength and the power of light entered.
The attenuation amounts of the respective ATTs 123 and 125 are so adjusted that the optical power of each channel in the C band and that in the L band are equalized at a point immediately downstream of the output point of the WDM coupler 126, that is, at point Y shown in FIG. 17.
In the above measurement system, WDM light beams having channels in the wavelength bands of the C band and the L band are generated, the optical powers are then adjusted on a wavelength band basis by the rare-earth-element-doped fiber amplifiers 122 and 124 and the ATTs 123 and 125, and resulting WDM light beams are wavelength-multiplexed into two-wavelength-band WDM light, which is sent to the SMF 127. Two-wavelength-band WDM light that has been transmitted through the SMF 127 over 80 km is measured by the optical spectrum analyzer 128.
Comparison between measurement results of FIGS. 18 and 19 shows that whereas in FIG. 18 the optical powers of the C band and the L band are approximately identical, in FIG. 19 (after transmission over 80 km) the optical powers of the C-band are smaller than those of the L band.
In FIGS. 18 and 19, the vertical axis represents the optical power in dBm and the horizontal axis represents the wavelength in nm. In FIG. 20, the vertical axis represents the Raman gain in dB and the horizontal axis represents the wavelength in nm.
FIG. 20 is a graph that is drawn based on FIGS. 18 and 19 to clarify the above finding. In FIG. 20, mark “x” represents optical powers that are obtained when only C-band WDM light s transmitted over 80 km, marks “▾” represent optical powers that are obtained only L-band WDM light is transmitted over 80 km, and marks “♦” represent optical powers that are obtained when both of C-band and L-band WDM light beams are transmitted over 80 km.
It is seen from FIG. 20 that when both of C-band and L-band WDM light beams are transmitted, the optical power of the C band decreases and the optical power of the L band increases, that is, the stimulated Raman scattering causes part of the power of the C band to be transferred to the L band.
The above measurement is directed to the case where C-band and L-band WDM light beams are transmitted in the same direction. A similar measurement was performed for a case where C-band and L-band WDM light beams are bidirectionally transmitted.
FIG. 21 shows a measurement system for the latter case. This measurement system is the same as the measurement system of FIG. 17 except that the block in FIG. 17 for generating L-band WDM light that is composed of the LDs 120-33 to 120-64, the AWG 120-2, the GS-EDFA 124, and the ATT 125 is provided on the side that is opposite, with respect to the SMF 127, to the side where C-band WDM light is generated and that an optical spectrum analyzer 130 for measuring a spectrum of L-band WDM light is added. Therefore, a description of the configuration of this measurement system is omitted.
In this measurement system, the optical power of generated C-band WDM light is adjusted by the EDFA 122 and the ATT 123 and resulting C-band WDM light is sent to the SMF 127. C-band WDM light that has been transmitted through the SMF 127 over 80 km is measured by the optical spectrum analyzer 128. On the other hand, the optical power of generated L-band WDM light is adjusted by the GS-EDFA 124 and the ATT 125 and resulting L-band WDM light is sent to the SMF 127. L-band WDM light that has been transmitted through the SMF 127 over 80 km is measured by the optical spectrum analyzer 130.
The ATTs 123 and 125 make such adjustments as to equalize the optical power of each channel in the C band at point Z1 (see FIG. 21) and that in the L band at point Z2 (see FIG. 21).
It is seen from a measurement result of FIG. 22 that the phenomenon that part of the optical power of the C band is transferred to the L band occurs in the same manner in the unidirectional transmission and the bidirectional transmission.
In FIG. 22, the vertical axis represents the Raman gain in dB and the horizontal axis represents the wavelength in nm. Marks “♦” represent optical powers in the unidirectional transmission that were transferred from FIG. 20 and marks “▪” represent optical powers in the bidirectional transmission.
It is seen from FIGS. 19, 20, and 22 that when two-wavelength-band WDM light is transmitted, stimulated Raman scattering causes part of the power of C-band WDM light to be transferred to L-band WDM light. That is, the stimulated Raman scattering causes part of the power of WDM light in a shorter-wavelength band to be transferred to WDM light in a longer-wavelength band. As a result, when n-wavelength-band WDM light is transmitted, optical power deviations occur between the wavelength bands and the optical SNRs of WDM light beams in shorter-wavelength bands are lowered.
In particular, as is understood from Equation (1), the optical SNRs deteriorate more in the case of ultra-long-distance transmission because Pi and Le become larger in that case.